The invention relates to motion-compensated noise evaluation in mobile wireless transmission systems, in which reference symbols are sent out by a transmitter.
Wireless transmission systems, as illustrated in FIG. 1, generally have at least one transmitter 1 as well as a receiver 2. In mobile transmission systems, the transmitter 1 or the receiver 2 is mobile, which is expressed in the figure by the mobile receiver 2. A data signal is sent out by the transmitter 1, which goes via a transmission channel, also known as a radio channel, to the receiver 2. The transmission properties of this transmission channel are dictated by the particular prevailing transmission situation. For example, the transmission situation is critically influenced by the distance between the transmitter 1 and the receiver 2, by reflections of data or radio signals from buildings, mountains, vehicles, or other obstacles 3, and also by the frequency shift caused by the Doppler effect when the relative speed between the transmitter 1 and receiver 2 is not insignificant.
Due to the influence of the transmission properties of the transmission channel, the data signal is distorted, so that the signal received in the receiver 2 generally does not match the data signal that was sent. It is therefore necessary to reconstruct the data signal in the receiver, which requires matching of the receiver to the transmission channel or its transmission properties in order to achieve the best possible reconstruction.
One of the major transmission properties of the transmission channel is its transfer function. The frequency spectrum of the signal received in the receiver is computed from the frequency spectrum of the data signal sent multiplied by this transfer function. Accordingly, the transfer function represents an interconnection between the data signal sent and the received signal. Thus, for example, evaluation of the transfer function is a central problem in coherent demodulation of Orthogonal Frequency Division Multiplexing (OFDM) signals.
The reconstruction of the data signal at the receiver end is typically impaired by noise and interference components, which are superimposed on the signal received in the receiver. The noise is caused primarily by thermal motions of elementary particles. The causes of interference are of a more diverse nature and can be produced, for example, by the simultaneous receiving of a foreign transmitter. The following will deal only with noise, understood as including the above-mentioned interference.
Moreover, a relative movement between the transmitter and the receiver also produces temporal variance or time variance, which is a change in the transmission channel or the transmission properties over time. This also impairs the reconstruction of the data signal at the receiving end.
Ensuring the highest possible quality of the data signal reconstructed at the receiving end therefore requires analysis of the received data signal in regard to the perturbing effects of noise and time variance of the transmission channel and a corresponding adapting of the receiver to the particular prevailing transmission properties.
For this reason, techniques are employed to evaluate the relative speed between a transmitter and a receiver, such as are known from German Patent DE 129 23 690 A1 or U.S. Pat. No. 6,636,574. Techniques are also known for estimating the noise level in transmission systems, whereby reference symbols are introduced into the data signal. Here, the noise level contained in the received data signal is estimated by the deviation of the received reference symbol from the reference symbol originally sent.
However, the noise components estimated in this way in the received data signal also contain interference components caused by the temporal change in the transmission channel or its transmission properties. When reconstructing the data signal originally sent using the results of the noise evaluation, these components of the time variance of the transmission channel lead to a distortion of the actual result. Furthermore, the noise components contained in the received data signal hinder a precise determination of the time variance of the transmission channel. In addition, a relative speed between the transmitter and the receiver is estimated, the result of this estimation is subject to errors by virtue of the noise.
Therefore, there is a need for a motion-compensated noise evaluation for mobile wireless transmission systems, in which reference symbols are sent out by a transmitter.